1. Field of the Invention
The present invention relates to a scattered radiation correction method and a scattered radiation correction method which are used on an X-ray diagnostic apparatus which can execute CT-like imaging, an X-ray computed tomographic apparatus or the like.
2. Description of the Related Art
In general, an X-ray projection image contains many scattered radiation components. This scattered radiation greatly degrades the accuracy of a CT value in three-dimensional imaging using a two-dimensional detector. A two-dimensional detector like a flat panel detector used in an X-ray diagnostic apparatus uses a scattered radiation removing grid to suppress scattered radiation. This effect, however, is much inferior to a scattered radiation removing effect in an X-ray computed tomographic apparatus. Scattered radiation correction is indispensable for extracting low-contrast information as of soft tissue by using three-dimensional imaging using a two-dimensional detector.
Scattered radiation is approximately modeled from primary X-rays passing through a subject to be examined. In practice, however, it is only possible to actually measure a composite image P′(x, y) comprising primary X-rays P(x, y) and scattered radiation S(x, y) like that represented by equation (1):P′(x,y)=P(x,y)+S(x,y)  <1>
In addition, the scattered radiation S(x, y) can be modeled like equation (2):S(x,y)={−P(x,y)log P(x,y)}*[Aexp{−(x2+y2)/(2a2)}+Bexp{−(x2+y2)/(2b2)}]  <2>where the symbol “*” represents a convolution operator. The term with the coefficient A is obtained by modeling Rayleigh scattering, and the term with the coefficient B is obtained by modeling Compton scattering. Scattered radiation correction is used to derive the primary X-rays P(x, y) from the composite image P′(x, y) according to equations (1) and (2).
It is, however, impossible to analytically calculate equations (1) and (2), and hence is impossible to directly obtain P(x, y). A conventional technique, therefore, calculates Pg(x, y) which minimizes equation (3) by a successive approximation method.E=|P′(x,y)−Pg′(x,y)|2  <3>where Pg′(x, y) is a composite image calculated based on Pg(x, y), which can be represented byPg′(x,y)=Pg(x,y)+Sg(x,y)  <4>
In this case, Sg(x, y) is written as follows:Sg(x,y)={−Pg(x,y)log Pg(x,y)}*[Aexp{−(x2+y2)/(2a2)}+Bexp{−(x2+y2)/(2b2)}]  <5>
Conventional scattered radiation correction, however, requires successive approximation calculation using equation (3) given above for each projection direction. For this reason, calculation processing requires much time.